Steering systems of many vehicles are equipped with an electric power steering device. The electric power steering devices are constructed to give a driver a comfortable steering feeling by alleviating necessary steering force to or effort be applied by the driver when rotating a steering wheel.
The electric power steering devices of this type include a rotational torque detection mechanism that is a device for, as a driver rotates the steering wheel, detecting intensity and direction of rotational torque (steering torque) acting on the steering system. Auxiliary or assisting torque is generated by an electric motor in accordance with results of the detection by the rotational torque detection mechanism, and the generated assisting torque is imparted to the steering system.
Particularly, the electric power steering devices of a construction where the assisting torque is transmitted to a steering shaft are called “column-assist-type electric power steering devices”. One example of such column-assist-type electric power steering devices is known from Patent Literature 1.
The column-assist-type electric power steering device known from Patent Literature 1 includes: a steering wheel; a steering shaft connected at one end to the steering wheel; two bearings rotatably supporting the steering shaft; and a torque sensor for detecting steering torque of the steering shaft. The torque sensor is disposed between one of the bearings that is adjacent to the steering wheel and the steering wheel, and a contact-type pressure sensor that is used as the torque sensor.
In recent years, non-contact-type sensors capable of measuring steering torque without contacting the steering shaft has been used more and more popularly. A magnetostrictive torque sensor is known as an example of the non-contact-type sensors.
In a case where the magnetostrictive torque sensor is employed in the electric power steering device disclosed in Patent Literature 1, it is necessary to provide a magnetostrictive film on the outer circumferential surface of the steering shaft but also provide a detection section in opposed relation to the magnetostrictive film for detecting variation in magnetic permeability of the magnetostrictive film. The detection section includes a pair of detection circuits for detecting variation in magnetic permeability of the magnetostrictive film.
However, the following problems would occur in the case where the magnetostrictive torque sensor is used.
For example, in putting the vehicle into a garage, a drive may sometimes slowly rotate the steering wheel to the fullest extent. If the driver attempts to further rotate the steering wheel from the fullest extent, a bending moment can act on the steering wheel and the steering shaft connected to the steering wheel.
An intermediate shaft is connected at an angle to the other end of the steering shaft via a universal joint. Thus, once an impact is input, in response to a vehicle's tire colliding against a curbstone, a bending moment may sometimes act on the steering wheel and the steering shaft from the universal joint as well.
These bending moments differ in intensity among various cross-sectional positions in an axial direction of the rotation shaft. More specifically, the intensity of the bending moment is zero at a point of action and gradually increases as the cross-sectional position gets away from the point of application. Thus, bending moments of different intensity act on magnetostrictive materials differing in the cross-sectional position and the point of action. For two magnetostrictive materials, both a rotational moment and a bending moment are detected as same strain. Namely, by detecting the rotational moment, accuracy of the magnetostrictive torque sensor for detecting the bending moment would decrease. Because bending moments of different intensity act on the two magnetostrictive materials, the bending moments cannot be canceled out even if a difference in output, such as AC resistance, from corresponding detection means is taken into account. As a consequence, detection accuracy of the steering toque would decrease. Therefore, there is still a room for further improvement.